od 
676 Physwgraphy. [ October, 
descending he entered the under current, which bore him safely 
back to Calais. 
The same laws are seen in operation in the Indian Ocean. 
There for half the year the North-east Monsoon which blows from 
the continent of Asia is the prevalent wind. During-the summer, 
however, it is forced back by a South-west wind, caused by the 
great upward draught over the glowing plains of Central Asia. 
Far away on the broad Atlantic and Pacific Oceans, we may see 
the same thing on a scale so magnificent as to form a healthy and 
vigorous circulation for the whole world. In the great system of 
winds, of which the trade winds are the most constant, we have 
mighty currents of air which sweep from pole to pole, and are the 
very life of the earth over which they pursue their ceaseless course. 
Thus the existence of the winds is due to sun-heat. 
Let us pause here fora moment to see what we have learnt. 
We have seen that the waves which beat on our shores, and de- 
nude our coast-lines, are due to the winds; that the rivers which 
cut down trenches into the earth are due to rain, which is itself 
brought to us as vapor of water by the winds; and we have seen 
that both the formation of water-vapor, and the existence of the 
winds, are due to sun-heat. This sun-heat is therefore the highest 
link we have yet reached in the chain of causation. We have also 
seen incidentally that the sand and clay at the top of the cliff were 
built up of mud and sand grains, carried down mechanically by 
rivers to the sea: and that the chalk has been separated by living 
creatures from the sea-water to which the lime had been carried 
down in solution by rivers. The question—how came this life 
upon the earth ?—now arises. It will not however be discussed 
here. It is enough to state that it is almost universally believed 
by those competent to give an opinion, that all life forms have 
come into being by a process of evolution from primitive organic 
germs. It may be noticed, however, that all life, whether vegeta- 
ble or animal, is made possible only by solar energy. Animals 
depend on plants, directly or indirectly, both for the food they eat 
and for the air they breathe. In the absence of sunlight plants 
would be unable to decompose the vast quantity of carbonic acid 
which animals breathe forth: and thus ż%eir source of carbon and 
| our source of oxygen would be cut o 
Another question must now be put and shortly answered. os 
3 Their direction i is modified by the rotation of the Earth. 
